ui/app: introduce DestroyEvent for ending the event loop

Replace the StageDead stage with DestroyEvent dedicated to ending
the event loop and, for premature window closes, the error.

Drop the error return from NewWindow; any errors in window creation
will appear as an immediate DestroyEvent with its Err field set.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/app.go

@@ -17,6 +17,8 @@ type Event interface {
 	ImplementsEvent()
 }
 
+// DrawEvent is sent when a Window's Draw
+// method must be called.
 type DrawEvent struct {
 	Config Config
 	Size   image.Point
@@ -27,6 +29,14 @@ type DrawEvent struct {
 	sync bool
 }
 
+// DestroyEvent is the last event sent through
+// a window event channel.
+type DestroyEvent struct {
+	// Err is nil for normal window closures. If a
+	// window is prematurely closed, Err is the cause.
+	Err error
+}
+
 // Insets is the space taken up by
 // system decoration such as translucent
 // system bars and software keyboards.
@@ -60,8 +70,7 @@ type windowAndOptions struct {
 }
 
 const (
-	StageDead Stage = iota
-	StagePaused
+	StagePaused Stage = iota
 	StageRunning
 )
 
@@ -93,8 +102,6 @@ var extraArgs string
 
 func (l Stage) String() string {
 	switch l {
-	case StageDead:
-		return "StageDead"
 	case StagePaused:
 		return "StagePaused"
 	case StageRunning:
@@ -104,10 +111,6 @@ func (l Stage) String() string {
 	}
 }
 
-func (_ DrawEvent) ImplementsEvent()     {}
-func (_ StageEvent) ImplementsEvent()    {}
-func (_ *CommandEvent) ImplementsEvent() {}
-
 func init() {
 	args := strings.Split(extraArgs, "|")
 	os.Args = append(os.Args, args...)
@@ -178,3 +181,8 @@ func newWindowRendezvous() *windowRendezvous {
 	}()
 	return wr
 }
+
+func (_ DrawEvent) ImplementsEvent()     {}
+func (_ StageEvent) ImplementsEvent()    {}
+func (_ *CommandEvent) ImplementsEvent() {}
+func (_ DestroyEvent) ImplementsEvent()  {}

ui/app/os_ios.go

@@ -111,7 +111,7 @@ func onStop(view C.CFTypeRef) {
 func onDestroy(view C.CFTypeRef) {
 	w := views[view]
 	delete(views, view)
-	w.w.event(StageEvent{StageDead})
+	w.w.event(DestroyEvent{})
 	C.gio_removeLayer(w.layer)
 	C.CFRelease(w.layer)
 	w.layer = 0

ui/app/os_macos.go

@@ -163,7 +163,7 @@ func getConfig() Config {
 func gio_onTerminate(view C.CFTypeRef) {
 	w := views[view]
 	delete(views, view)
-	w.setStage(StageDead)
+	w.w.event(DestroyEvent{})
 }
 
 //export gio_onHide

ui/app/os_wayland.go

@@ -113,6 +113,7 @@ type window struct {
 	lastTouch f32.Point
 
 	stage             Stage
+	dead              bool
 	lastFrameCallback *C.struct_wl_callback
 
 	mu        sync.Mutex
@@ -308,7 +309,8 @@ func gio_onXdgSurfaceConfigure(data unsafe.Pointer, wmSurf *C.struct_xdg_surface
 //export gio_onToplevelClose
 func gio_onToplevelClose(data unsafe.Pointer, topLvl *C.struct_xdg_toplevel) {
 	w := winMap[topLvl]
-	w.setStage(StageDead)
+	w.dead = true
+	w.w.event(DestroyEvent{})
 }
 
 //export gio_onToplevelConfigure
@@ -769,7 +771,7 @@ loop:
 		if ret := C.wl_display_flush(conn.disp); ret < 0 {
 			break
 		}
-		if w.stage == StageDead {
+		if w.dead {
 			break
 		}
 		// Clear poll events.

ui/app/os_windows.go

@@ -61,6 +61,7 @@ type window struct {
 	width  int
 	height int
 	stage  Stage
+	dead   bool
 
 	mu        sync.Mutex
 	animating bool
@@ -319,7 +320,8 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 		w.scrollEvent(wParam, lParam)
 	case _WM_DESTROY:
 		delete(winMap, hwnd)
-		w.setStage(StageDead)
+		w.dead = true
+		w.w.event(DestroyEvent{})
 	case _WM_REDRAW:
 		w.mu.Lock()
 		anim := w.animating
@@ -368,7 +370,7 @@ func (w *window) scrollEvent(wParam, lParam uintptr) {
 // Adapted from https://blogs.msdn.microsoft.com/oldnewthing/20060126-00/?p=32513/
 func (w *window) loop() error {
 loop:
-	for w.stage > StageDead {
+	for !w.dead {
 		var msg msg
 		// Since posted messages are always returned before system messages,
 		// but we want our WM_REDRAW to always come last, just like WM_PAINT.

ui/app/window.go

@@ -29,7 +29,6 @@ type Window struct {
 	drawStart  time.Time
 	gpu        *gpu.GPU
 	inputState key.TextInputState
-	err        error
 
 	events chan Event
 
@@ -70,7 +69,7 @@ var ackEvent Event
 // ignore or adjust them.
 // If the current program is running on iOS and Android,
 // NewWindow returns the window previously by the platform.
-func NewWindow(opts *WindowOptions) (*Window, error) {
+func NewWindow(opts *WindowOptions) *Window {
 	if opts == nil {
 		opts = &WindowOptions{
 			Width:  ui.Dp(800),
@@ -87,9 +86,11 @@ func NewWindow(opts *WindowOptions) (*Window, error) {
 		stage:  StagePaused,
 	}
 	if err := createWindow(w, opts); err != nil {
-		return nil, err
+		// For simplicity, NewWindow always succeeds. Send
+		// an immediate DestroyEvent instead of returning the error.
+		w.destroy(err)
 	}
-	return w, nil
+	return w
 }
 
 func (w *Window) Events() <-chan Event {
@@ -107,10 +108,6 @@ func (w *Window) setTextInput(s key.TextInputState) {
 	w.inputState = s
 }
 
-func (w *Window) Err() error {
-	return w.err
-}
-
 func (w *Window) Queue() input.Queue {
 	return &w.router
 }
@@ -144,12 +141,12 @@ func (w *Window) Draw(root *ui.Ops) {
 	if w.gpu == nil {
 		ctx, err := newContext(driver)
 		if err != nil {
-			w.err = err
+			w.destroy(err)
 			return
 		}
 		w.gpu, err = gpu.NewGPU(ctx)
 		if err != nil {
-			w.err = err
+			w.destroy(err)
 			return
 		}
 	}
@@ -190,6 +187,9 @@ func (w *Window) updateAnimation() {
 		w.delayedDraw.Stop()
 		w.delayedDraw = nil
 	}
+	if !w.isAlive() {
+		return
+	}
 	if w.stage >= StageRunning && w.hasNextFrame {
 		if dt := time.Until(w.nextFrame); dt <= 0 {
 			animate = true
@@ -223,7 +223,7 @@ func (w *Window) Stage() Stage {
 }
 
 func (w *Window) isAlive() bool {
-	return w.stage > StageDead && w.err == nil
+	return w.driver != nil
 }
 
 func (w *Window) contextDriver() interface{} {
@@ -236,10 +236,18 @@ func (w *Window) setDriver(d *window) {
 	w.driver = d
 }
 
+func (w *Window) destroy(err error) {
+	w.setDriver(nil)
+	go func() {
+		w.event(DestroyEvent{err})
+	}()
+}
+
 func (w *Window) event(e Event) {
 	w.eventLock.Lock()
 	defer w.eventLock.Unlock()
 	w.mu.Lock()
+	died := false
 	needAck := false
 	switch e := e.(type) {
 	case input.Event:
@@ -248,12 +256,13 @@ func (w *Window) event(e Event) {
 		}
 	case *CommandEvent:
 		needAck = true
+	case DestroyEvent:
+		w.driver = nil
+		died = true
 	case StageEvent:
 		w.stage = e.Stage
-		if w.stage > StageDead {
-			needAck = true
-			w.syncGPU = true
-		}
+		needAck = true
+		w.syncGPU = true
 	case DrawEvent:
 		if e.Size == (image.Point{}) {
 			panic(errors.New("internal error: zero-sized Draw"))
@@ -290,7 +299,7 @@ func (w *Window) event(e Event) {
 			w.gpu.Refresh()
 		}
 	}
-	if stage == StageDead {
+	if died {
 		close(w.events)
 	}
 }